A body of a motor vehicle or the like is affixed with a vibration/sound insulation material, namely, a damping material (asphalt sheet etc.) in order to reduce interior noise of the vehicle. When affixing the damping material to the vehicle body, it is customary to set a sheet-like material onto the vehicle body and bond the material to the vehicle body by applying heat thereto in a paint baking process.
To enhance the rigidity and damping effect of the damping material, it is also customary to overlay the damping material with a sheet-like constraining material (thermosetting resin sheet etc.), which is a kind of damping material.
In the case where such sheet-like materials are used as the damping or constraining material, however, it is necessary to prepare various sheets matching the shapes etc. of vehicle bodies, and also a problem arises in that much time and labor are required to lay the various sheets on the vehicle bodies.
In view of this, Examined Japanese Patent Publication No. Sho 56-23662 discloses a technique of applying a viscous damping material to a vehicle body to provide the vehicle body with the damping material.
According to the technique disclosed in this publication, however, the vehicle body is applied only with a single layer of viscous damping material. Such a single layer of damping material can provide a damping effect to a certain degree, but the available damping performance is inferior to that achieved by the aforementioned double sheet structure of damping and constraining materials.
After a viscous damping material is applied to the vehicle body, a viscous constraining material may be applied over the damping material. In this case, however, the application of both the damping and constraining materials needs to be completed in a very short working time. Moreover, there arises a problem of how efficiently the damping and constraining materials, both having viscosity, should be applied to the vehicle body in a so-called Wet-On-Wet state while at the same time ensuring sufficient damping performance.
To enable efficient application of the damping and constraining materials to the vehicle body while ensuring sufficient damping performance, a problem of what compositions should be selected for the damping and constraining materials needs to be solved. The selection of the constraining material, in particular, is essential because the material is required to have the following characteristics among others: (1) The constraining material should exhibit sufficiently high rigidity and the effect of constraining the damping material in an operating temperature range. (2) The constraining material should not hinder the drying of the underlying damping material and yet should dry and harden itself with heat applied during the paint baking process. (3) The constraining material should have such a viscosity characteristic as to permit the damping and constraining materials to be simultaneously applied during the application process.